<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <div :class="a">div1</div>
    <div :class="['div', a]">div2</div>
    <div :class="{a: 1==1}">div3</div>
    <div :class="['div',a, {active: 2==2}]">div4</div>
  </div>
  <script src="./vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
        a:'box'
      }
    })
  </script>
</body>
</html>